Saving money and lives, as well as improving national security, are the goals behind a U.S. military initiative to reduce its reliance on fossil fuels. At one outpost in remote Afghanistan, Marines are using solar power; the Navy is also experimenting with biofuels.

Harbor porpoises are showing up in growing numbers under the Golden Gate Bridge. Six decades ago, they were driven away in part by wartime activities and poor water quality; now, researchers are trying to understand why they're returning.

Nuclear power generates 20 percent of electricity in the U.S., but the nation's reactors are aging — and new plants are expensive and take years to build. Gas, coal, wind and solar are potential alternatives, but all have environmental or logistical drawbacks.

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